An RC filter (or RC network) is a common circuit unit used in a wide variety of different integrated circuits (ICs), particularly analog circuits. One example is for realizing a low pass (LP) filter to filter out high frequency noise, suppress electromagnetic interference (EMI), or to provide a more stable output signal. However, RC filters can also be configured as high-pass filters. The traditional way to form a first order (i.e., single pole) RC filter is use two discrete devices side-by-side on the die, including a separate R and a C hooked up in series, and in the case of a low pass filter to take the RC filter's output across the plates of the C.
Often the resistance of the R needs to be a high value, typically in a range from tens of ohms to tens of mega (106) ohms. Sheet resistance (in ohms/square (sq)) is a R area independent resistance measurement used to characterize different materials that is inversely related to the thickness of the R layer. The sheet resistance varies significantly for different materials, from several milli-ohm/sq for a metal layer to tens of kilo-ohm/sq for an undoped polysilicon layer. When one knows the shape of the R (thus its length and width), the number of squares is known, which enables using the sheet resistance multiplied by the number of squares to calculate the resistance value of the R.